1. Field of the Invention
This invention relates to a multihead sewing machine including a sewing machine motor for driving a main shaft further driving a plurality of sewing needles, and a plurality of shuttle driving motors for driving a plurality of shuttles for capturing thread loops in cooperation with the sewing needles respectively.
2. Description of the Related Art
There have conventionally been provided vertical axis drive type sewing machines comprising a single sewing machine motor for driving both a main shaft for vertically driving sewing needles and thread take-up levers, and a lower shaft for driving thread loop capturing shuttles such as full-turn shuttles. On the other hand, Japanese patent publication Nos. 60-21750-B and 3-234291-A each disclose a hook shaft independent drive type sewing machine comprising an independent shuttle driving motor for driving a thread loop capturing shuttle as well as a sewing machine motor for driving a main shaft. In the independent hook shaft drive type sewing machines, the sewing machine motor and the shuttle driving motor are controlled so that the shuttle is driven in synchronism with the main shaft of the machine. Furthermore, the shuttle is controlled to be driven according to various sewing conditions.
In the hook shaft independent drive type sewing machines, various countermeasures have been proposed to prevent loss of synchronism between the needles and the shuttles when the main shaft is manually rotated. However, no technique has been proposed to detect an amount of displacement due to the loss of synchronism caused during the sewing operation. Furthermore, no countermeasures have been known to be taken when the amount of displacement has exceeded an allowed value. In view of these circumstances, the assignee of the present invention filed a patent application in Japan for the invention relating to a sewing machine provided with the above-described countermeasures, and the application was assigned Japanese patent application No. 8-85917.
The above-mentioned Japanese patent application No. 8-85917 discloses a multihead embroidery machine provided with three sewing machine units and comprising a sewing machine motor for driving a main shaft further driving sewing needles of the sewing machine units simultaneously, thread loop capturing shuttles provided in cylindrical sewing beds of the heads, and shuttle driving motors provided in the sewing beds for driving hook shafts of the shuttles, respectively. In this construction, each hook shaft is controlled to be driven in synchronism with rotation of the main shaft in a normal sewing. In a case where thread jamming etc. desynchronizes any one of the three shuttles during the sewing, loss of synchronism is detected when an amount of displacement of the hook shaft with respect to the main shaft reaches a predetermined value. Upon detection of the loss of synchronism, emergency stop of the machine motor is effected, and a needle bar jumping mechanism is actuated to jump a needle bar of each head up to its uppermost position. Furthermore, each shuttle driving motor is stopped. Consequently, the sewing needles can be prevented from colliding against the respective shuttles.
In the above-described multihead embroidery machine, the sewing machine motor and the shuttle driving motors differ from one another in a driving force, load inertia, etc. Accordingly, the machine motor and each shuttle driving motor cannot be stopped in emergency simultaneously. Furthermore, a timing for jumping each needle bar lags behind. Consequently, loss of synchronism of each hook shaft with respect to the main shaft may result in collision of each needle against the corresponding shuttle in the emergency stop. This poses a problem.
In the above-described multihead embroidery machine, furthermore, the emergency stop sometimes causes the main shaft and each shuttle to assume respective rotational positions which are not predetermined ones for the start of the sewing, for example, respective positions at which a needle thread loop is formed on each shuttle with the thread take-up lever remaining in the lowered state. In this case, a manually operated pulley is driven so that the main shaft is rotated to assume a sewing start position which is away about 100 degrees from an original position, the sewing start position at which the thread take-up lever is lifted so that the thread is tightened up. Since the shuttle driving motor is stopped, the thread take-up lever is lifted up with the needle thread loop being formed on the shuttle, whereupon the needle thread loop remains below a throat plate. Thereafter, upon instruction of re-start of the sewing, initializing processes are executed in which each shuttle is rotated to assume the sewing start position, and subsequently, the sewing is re-started. However, the needle thread loop remaining below the throat plate may be caught by the rotating shuttle. This would result in thread jamming or prevent rotation of the shuttle, posing a problem.